Mechanical properties of crystalline and amorphous aluminum oxide thin films grown by atomic layer deposition

Mechanical properties of atomic-layer deposited alumina thin films containing amorphous, alpha-Al2O3, theta-Al2O3, delta-Al2O3 and gamma-Al2O3 phases were characterized. To initiate the alpha-Al2O3 growth, alpha-Cr2O3 seed layers deposited on Si substrates were used. Application of AlCl3 and H2O as the most appropriate precursors together with sufficiently long precursor pulse durations appeared to be important to obtain alpha-Al2O3 at temperatures down to 350 degrees C. The highest hardness value measured by the nano-indentation method for as-grown films was 26 GPa. This value was recorded for films deposited on alpha-Cr2O3 at 450 degrees C. Post-growth annealing of these films at 750 degrees C caused a hardness increase up to 30 GPa, that is, close to the corresponding value of single crystal alpha-Al2O3. For comparison, the hardness values of amorphous films did not exceed 13 GPa while those of the films, containing theta-Al2O3 , delta-Al2O3 and gamma-Al2O3 phases grown on pristine Si substrates at 750 degrees C reached 18 GPa. The increase in hardness and elastic modulus was shown to be in correlation with the increase in crystallinity and density of the film material.